1. Field of the Invention
The present invention relates to an optical recording medium having a recording track and a wobbling groove formed along the recording track in which a signal is recorded as a reflection ratio change of a recording material based on an organic dye while reproducing a wobble signal from the wobbling groove.
2. Description of the Prior Art
As a recording medium for storing information, an optical disc has been used in practice for optically recording/reproducing information. In such an optical disc, information is recorded/reproduced with a head in a non-contact state and according no problems arise such as abrasion or deterioration of the recording medium. For this, the optical disc is now widely used as a recording medium having a high reliability.
Among such optical discs, a so-called heat mode optical disc has been developed in which a user can write information by utilizing a heat of the light applied to the optical disc. As an example, an optical disc of a so-called pit formation type is now used in practice. A recording light such as a laser light is applied to the optical disc to cause a heat which forms a small pit in the recording material, thereby recording information.
In this pit formation type optical disc, generally, the recording material used is a chalcogen material containing tellurium (Te) as a main content or an organic dye-based material. These recording materials exhibit a high sensitivity and a low noise and are little deteriorated by lapse of time. Recently, especially, the organic dye-based material is widely used.
As an optical disc using an organic dye-based material as the recording medium, an optical disc (hereinafter, referred to as a CD-R) used for a so-called compact disc recordable system is now used in practice.
The CD-R has a wobbling groove formed along the recording track. Here, the wobbling groove is a guide groove formed with a predetermined meander so that the guide groove itself has a signal component. It should be noted that the guide groove is a groove formed along the recording track so as to facilitate tracking servo by, for example, the push-pull method.
In the CD-R, a sector information containing an absolute time information which has been subjected to FM modulation is recorded as a signal (wobble signal) by this wobbling groove (ATIP: Absolute Time In Pregroove). That is, in the compact disc recordable system using the CD-R as the recording medium, a recording/reproducing light spot focused on the wobbling groove detects a wobble signal using, for example, 22.05 kHz as a carrier and detects a data string containing an absolute time information by FM demodulation of the signal.
In this method recording as a wobble signal a sector information containing an absolute time information, it is possible to continuously record signals, which is advantageous for compatibility with a ROM optical disc in which signals are continuously recorded. That is, in the method in which an address information is arranged at a head of each sector, an absolute time information and a recording signal are recorded by time division method, signals recorded become discontinuous, causing difficulty for compatibility with a ROM optical disc in which signals are continuously recorded. In contrast to this, in the method in which a sector information containing an absolute time information is recorded as a wobble signal, it is easy to obtain compatibility with a ROM optical disc.
In the aforementioned optical discs, various trials are now performed to increase the recording density so as to record more information within the existing outer diameter of the normalized optical discs. For example, in the aforementioned CD-R, an attempt is being made to increase the recording density so as to realize a recording capacity equal to or above 1.0 GB (gigabytes).
When considering the higher recording density, it is effective to reduce the track pitch as an interval between adjacent recording tracks. However, as the track pitch is reduced, more information leaks from adjacent recording tracks, increasing a signal jitter component.
Moreover, in an optical disc having a wobbling groove along the recording track in which an absolute time information or the like is recorded as a wobble signal, if the track pitch of the recording track is reduced, the more wobble signal leaks from the adjacent wobbling grooves, increasing the wobble signal jitter component. When the wobble signal jitter component is significantly increased, it becomes difficult to assure to recognize the time information and the like contained in the wobble signal, disabling to obtain stable information recording/reproducing.
In the optical disc using an organic dye-based material as the recording material, the wobble signal jitter characteristic greatly depends on the state of the organic dye-based recording material filled in the wobbling groove. In other words, the wobble signal jitter characteristic is greatly changed by the viscosity and wettability of a paint liquid applied on the substrate for forming the recording layer.
The viscosity and the wettability are determined by the feature inherent to the dye used. Accordingly, when the recording material can use a dye having a feature appropriate to suppress the wobble signal jitter component, even in the optical disc having a reduced track pitch to obtain a higher recording density, it is possible to suppress the wobble signal jitter component to a sufficiently low value practically.
It is therefore an object of the present invention to provide an optical recording medium realizing a high recording density while suppressing a wobble signal jitter component to a sufficiently low value so as to obtain stable recording/reproducing of information.
The optical recording medium according to the present invention is an optical recording medium having a recording track in which a signal is recorded as a reflection ratio change of an organic dye-based recording material. In this optical recording medium, a wobbling groove is formed along the recording track, the recording track has a track pitch of 1.3 micrometers or below, and the recording material contains at least one of compounds shown in Chemical Formula 5 to 7 below: 
wherein R1 and R2 are alkyl groups selected from a group consisting of n-C4H9, n-C3H7, and n-C5H11, and X1xe2x88x92 is an anion; 
wherein R1 and R2 are alkyl groups selected from a group consisting of n-C4H9, n-C3H7, and n-C5H11, and X1xe2x88x92 is an anion; 
wherein R1 and R2 are alkyl groups selected from a group consisting of n-C4H9, n-C3H7, and n-C5H11, and X1xe2x88x92 is an anion.
Alternatively, in the optical recording medium according to the present invention, a wobbling groove is formed along a recording track, the recording track has a track pitch of 1.3 micrometers or below, and the recording material contains a compound shown in Chemical Formula 8 below: 
wherein R1 and R2 are alkyl groups selected from a group consisting of n-C4H9, n-C3H7, and n-C5H11, and X2xe2x88x92 is an anion (excluding ClO4xe2x88x92)
In the optical recording medium according to the present invention uses the material containing the compounds shown in the aforementioned Chemical Formulae 5 to 8 and accordingly, even when the track pitch is reduced to 1.3 micrometers or below, it is possible to suppress the jitter component of the wobble signal required for recording/reproducing to a sufficiently low value for practical use. Consequently, in this optical recording medium, it is possible to realize a high recording density while obtaining stable recording/reproducing of information.